(a) Field of the Invention
The present invention relates to an adhesive composition for metal-clad laminates, which can be used for bonding a metal foil to a laminate.
(b) Description of the Related Art
With the advances of the minimization and functional improvement of electronic equipments for domestic use, the printed wiring boards to be used for such electronic equipments have become denser and minuter in their wiring patterns. Following this there has been an increasing demand for metal-clad laminates which can bring about printed wiring boards capable of enabling high density packaging. Therefore, the requirements for the solder resistance and peel strength have become more strict. Additionally, for those on which a high voltage may be applied, for example, those for televisions, tracking resistance has become essential in order to insure security. "Tracking" is a phenomenon that the surface of an insulator is carbonized in the regions on which potential differences generate, thereby forming conductive paths.
In the past, adhesives prepared by blending a polyvinylbutyral with a phenolic resin have been used for bonding a metal foil to a laminate because they have excellent soldering heat resistance and metal peel strength.
Though such adhesives prepared by blending a polyvinylbutyral with a phenolic resin excel in soldering heat resistance and metal peel strength, they have poor tracking resistance because phenolic resins contained therein are easily carbonized causing continuity.
In Japanese Patent Application Kokai Koho (Laid-open) No. 02-110082, etc. proposed was the use of epoxy resins or melamine resins, which are hardly carbonized, as the substituent component for phenolic resins. Although an increase in tracking resistance was attained in these methods, it was difficult to maintain the solder resistance at the required level.
Further, a high peel strength of circuits, i.e. a high peel strength, at high temperatures has come to be required because circuits generate heat when a high voltage is applied thereon. However, no adhesive has ever succeeded in satisfying both the requirements for the tracking resistance and the peel strength at high temperature.